Thousands of fish have mysteriously washed up on the shores of Burrinjuck Dam, north-west of Canberra.
Liz Richardson from Good Hope Tourist Resort told The Yass Tribune the amount of dead fish was "unbelievable".
"Every type of fish," she said. "Lots of baby fish, lots of small cod."
Around eight kilometres of shoreline, downstream of Taemas Bridge and past Good Hope, was littered with fish carcasses.
The Department of Primary Industries said the cause of the fish kill was unclear.
Fisheries officers inspected the site on Tuesday and observed hundreds of native golden perch and Murray cod among the thousands of dead fish.
"Sizes of fish affected ranged from five to 80 centimetres.
"The fish were observed floating in the water, along the water's edge and on the shoreline over an area of approximately eight kilometres within the impoundment," a DPI spokesman said.
Ms Richardson said a similar thing happened about 10 years ago when the water levels were low, and heavy rain had fallen.
She believed the deaths were due to the rapidly falling dam levels, which, combined with recent storm run-off, may have created low oxygen supplies in the dam.
"They've taken the water so dangerously low for the fish... they've ravaged the water," she said.
"In three-and-a-half months [the lake's] gone from 100 per cent to 38 per cent [full].
"We've had big rain before," she said. "Like last year with the floods. It's just the fact they've let all the water out."
Dean Brind from Burrinjuck State Park said the falling water level shouldn't have killed the fish.
"The fish normally will keep moving down unless they get stuck in pockets," Mr Brind said.
He was told the first dead fish appeared Saturday morning before the rain. He didn't know the cause but ruled out several factors.
"It's not cold water, it's not run off from the fires," he said, as the fire run-off would have washed into the dam from the Yass River area.
Officers from the Environment Protection Authority (EPA) took water samples from Hume Park on Wednesday.
"Fisheries have let us know there's been a fish kill," head of operations, Sandie Jones, said.
"When there's no clear cause we'll come out to take samples."
Hume Park management was worried about people wanting to use the dam.
"[There's] concern as to why it has occurred and whether or not it is safe for recreational use."
Resort owners will be rolling up their sleeves to clean up the dead fish in their immediate area. It is not known who is responsible for the clean-up of the rest of the affected area.
Fishing takes a hit
Thousands of cod fingerlings released recently may all lay dead on the banks of Burrinjuck Dam.
Mr Brind from Burrinjuck State Park said around 65,000 cod were released before Christmas and just last week there was a trout release.
"Most of them were in that stretch of river," he said. "That's probably wiped those two releases out.
"That's a year we've lost."